1. Field of the Invention
The present invention relates to a laser light irradiation apparatus and a laser light irradiation method, and particularly relates to a laser light irradiation apparatus and a laser light irradiation method using a beam expander optical system.
2. Description of the Related Art
Recently, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has been drastically advanced and developed for being applied to an active matrix display device. In particular, a TFT using a polycrystalline semiconductor film has a higher electron field-effect mobility (also referred to as a mobility) than a conventional TFT using an amorphous semiconductor film, so that high-speed operation is possible. Therefore, the control can be carried out by a driver circuit which is formed over the same substrate on which a pixel portion is formed, although the control of a pixel portion has been conventionally carried out by a driver circuit which is provided outside a substrate on which the pixel portion is formed.
As a substrate used for a semiconductor device, a glass substrate is expected more than a quartz substrate and a single-crystalline semiconductor substrate in terms of cost. However, the glass substrate has poor heat resistance and is easily deformed by heat. Therefore, when a semiconductor film is crystallized in order to form a TFT using a polycrystalline semiconductor film over a glass substrate, a method for crystallizing a semiconductor film by laser light irradiation is often used to avoid thermal deformation of the glass substrate.
Features of the crystallization of a semiconductor film by laser light are that, compared with an annealing method utilizing radiation heating or conductive heating, processing time can be drastically reduced, and a semiconductor substrate or a semiconductor film over a substrate is selectively or locally heated so that the substrate is hardly thermal-damaged, for example.
Generally, laser light (also referred to as a laser beam) oscillated from a laser oscillator has a Gaussian spatial intensity distribution. Therefore, in the case where an irradiated object is directly irradiated with laser light oscillated from a laser oscillator, the energy distribution varies in an irradiated region. For example, when crystallization or improvement of film quality is conducted by irradiating a semiconductor film including silicon or the like with laser light, if the semiconductor film is directly irradiated with laser light having a Gaussian spatial intensity distribution, the energy distribution is different in a central portion and an edge portion of an irradiated region, so that melt time of the semiconductor film varies. Consequently, crystallinity of the semiconductor film is nonuniform, and a semiconductor film having a desired characteristic cannot be obtained.
Accordingly, in general, after the spatial intensity distribution of laser light oscillated from a laser oscillator is uniformed by using some kind of laser light shaping means, an irradiated object is irradiated with the laser light. For example, as the laser light shaping means, a beam expander optical system is widely used (for example, see Patent Document 1: Japanese Published Patent Application No. H741845)
A conventional beam expander optical system is, as illustrated in FIG. 5, composed of two lenses 1102a and 1102b, which are disposed so that an optical distance from lens 1102a to lens 1102b is f1+f2 when the focal length of lens 1102a and 1102b are f1 and f2, respectively. Accordingly, laser light 1105 emitted from a laser oscillator 1101 passes through a beam expander optical system 1102 to be expanded to f2/f1 times and projected onto an irradiated surface. At this time, for example, a diffractive optical element 1104 is disposed behind the beam expander optical system 1102, so that a desired shaped laser light can be obtained.
Since a diffractive optical element is generally an element which has a minute and complicated structure, it is necessary to make laser light enter the extremely proper position at the diffractive optical element. Reduction in a diameter of a diffractive optical element is extremely difficult at present; therefore a method in which laser light is propagated to the diffractive optical element after the laser light is expanded by the beam expander optical system or the like as described above is employed.